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Femtosecond laser micro-drilling has progressed remarkably in recent years to become an essential tool for micro-hole drilling in many applications where the extreme high accuracy machining is required. This paper addresses the problem of finding optimal inspection policy in a femtosecond laser micro-machining process for micro-hole array products to improve successful rate of machined parts and hence to minimise the unit manufacturing cost. A solution algorithm is proposed based on particle swarm optimisation (PSO), and the simulation is used to provide better insight into the optimal solution. The numerical experiments and verification experiments which are conducted with different schemes illustrate the applicability and efficiency of the proposed approach. [ABSTRACT FROM AUTHOR]

Abstract: Most of reported works on laser micro-machining have been carried out under ‘tight-focusing’ arrangement. However, defocusing has to be considered when dealing with depth evolution in multiple pulse ablation. In this paper, we show that laser intensity distribution is very sensitive to defocusing and, in multiple pulse ablation, defocusing is unavoidable even under the ‘tight focusing’. Both theoretic analysis and experimental result indicate that convergent defocusing is a better option for some micro-machining purposes. The optimal defocusing arrangement not only results in a maximum material removal rate (MRR) and maximum depth evolution, but also prevents harmful heat damage caused by extreme high intensity at the focal center. [Copyright &y& Elsevier]

Abstract: The derived light from laser triggered plasma is used as an indicator for a closed-loop control in the femtosecond pulse laser micro-milling. The relationship between the brightness of derived light and the defocusing distance is pre-measured as a basic curve for the closed-loop control milling to assure the machining accuracy. The two elements, i.e. the defocusing distance and the size of the laser affected volume, are analyzed to predict the final machining result. By monitoring the brightness of the derived light, the ablation depth during a femtosecond pulse laser micro-machining process can be controlled in a real-time manner. A micro-milling experiment as reported in this paper demonstrates the feasibility and efficiency of this novel closed-loop micro-machining method. [Copyright &y& Elsevier]

Abstract: The measurement of the ablation threshold intensity (ATI) is essential for an ultra-fast laser micro-machining process. Unlike the widely used Liu''s method, the method presented in this paper investigates the laser Affected Volume and demonstrates that, on some part of the boundary of the affected volume, the laser intensity is equal to the ATI. By defocusing, the size and the shape of the single pulse laser affected volume can be detected to measure the ATI. The potential to setup a standard database of the ATI of various materials by simply calibrating the samples with a benchmark material is also discussed. The method is practically useful as an in situ measurement method in femtosecond pulse laser micro-machining applications. [Copyright &y& Elsevier]

The plasma brightness cannot be used as a direct indicator of ablation depth detection by femtosecond laser was experimentally demonstrated, which led to the difficulty of depth measurement in the maching process. The tests of microchannel milling on the silicon wafer were carried out in the micromachining center in order to obtain the influences of parameters on the ablation depth. The test results showed that the defocusing distance had no significant impact on ablation depth in LAV effective range. Meanwhile, the reason of this was explained in this paper based on the theoretical analysis and simulation calculation. Then it was proven that the ablation depth mainly depends on laser fluence, step distance and scanning velocity. Finally, a research was further carried out to study the laser parameters which relate with the microchannel ablation depth inside the quartz glass for more efficiency and less cost in processing by femtosecond laser. [ABSTRACT FROM AUTHOR]